1. Field of the Invention
The present invention relates to a power amplifying device, and more particularly, to a power amplifying device having a linearizer, in which a bias circuit has an initial impedance set when initially operated, then the impedance is varied according to a level of an input signal and the input signal is amplified in a broad range from a low level region to a high level region, thereby improving linearity of an output signal.
2. Description of the Related Art
In general, a power amplifying device essentially employed in a wireless telecommunication system, when operating in a saturation region having nonlinear characteristics, experiences distortion in size and phase of an output signal due to the nonlinear characteristics, thereby generating signal distortion components. In consequence, this significantly degrades transmission performance of the telecommunication system.
To reduce these inter-modulation distortion components, the power amplifying device may linearly amplify an output while ensuring high linearity. The power amplifying device can perform linearization via a back-off method or by employing a bias structure using a linearizer.
First, when it comes to the back-off method, the power amplifying device, when used at a P1 dB band, i.e., a point where the output signal is 1 dB low from a saturation region, undergoes severe non-linearity, and thus the output of the power amplifying device is backed off from the P1 dB band to several dB. However, in this back-off method, the power amplifying device is operated in a several dB low region, which is not a maximum output region having maximum efficiency. This as a result considerably undermines efficiency of the power amplifying device.
Next, a description will be given of a method of employing a bias structure using a linearizer in a power amplifying device with reference to FIGS. 1A and 1B.
FIG. 1A is a circuit diagram illustrating an example of a conventional power amplifying device.
Referring to FIG. 1A, the conventional power amplifying device 10 includes a signal amplifying unit 11, a bias unit 12, a linearizing unit 13 and a current compensating unit 14. The signal amplifying unit 11 has a transistor amplifying an input signal RF IN in response to an operating power source (Vcc). The bias unit 12 has a transistor supplying a bias current to the signal amplifying unit 11. The linearizing unit 13 has a passive capacitor and the current compensating unit 14 compensates for a current supplied to the bias unit.
The transistor of the signal amplifying unit 11 receives the operating power sources and operates in response to the operating power source and then amplifies the input signal input signal (RF IN) fed through a base terminal. Here, the transistor of the bias unit 12 has an impedance lower than the transistor of the signal amplifying unit 11. Thus, in a case where the input signal is a high frequency signal, the input signal RF IN is fed to the transistor of the bias unit 12, thereby decreasing potential between a base and emitter of the transistor of the bias unit 12. This accordingly compensates for decrease in the base-emitter voltage of the transistor of the signal amplifying unit 11, thereby improving linearity of an output signal RF out.
The conventional power amplifying device 10 as described above is structured such that a passive capacitor of the linearizing unit 13 rectifies the input signal RF IN and generates an additional direct current (DC) in response to the input signal. Here, the passive capacitor has a capacitance fixed. Thus, in a case where the input signal RF IN has a low level, the fixed capacitance leads to a low impedance, thereby increasing current consumption and degrading efficiency. This also aggravates inter-modulation distortion.
FIG. 1B is a circuit diagram illustrating another example of a conventional power amplifying device.
Referring to FIG. 1B, the conventional power amplifying device 20 has a linearizing unit 23 structured differently from that of the power amplifying device 10 shown in FIG. 1A. The conventional linearizing unit 23 of the conventional power amplifying device 20 includes a reverse diode. The reverse diode supplies a dynamic impedance to a transistor of a bias unit 22 so that the input signal (RF IN) is less applied to the bias unit 22 in a low level region and the input signal is more applied to the bias unit 22 in a high level region. This as a result improves linearity of an output signal RF OUT generated from the amplified input signal.
Here, due to presence of equivalent capacitance in the low level region, a portion of the input signal (RF IN) is applied to the linearizing unit 23 via the bias unit 22, accordingly increasing current consumption, undermining efficiency and aggravating inter-modulation distortion.